Sintered alloys of a chromium carbide-tungsten carbide-nickel system

ABSTRACT

SINTERED ALLOYS MAINLY CONSISTING OF 25 TO 75 PERCENT BY WEIGHT OF CHROMIUM CARBIDE CR2C2, 10 TO 43 PERCENT BY WEIGHT OF TUNGSTEN CARBIDE WC AND 15 TO 50 PERCENT BY WEIGHT OF NICKEL NI AND CONTAINING ONE PERCENT MAX. BY WEIGHT OF IMPURITES. THESE ALLOYS ARE FULLY RESISTANT TO CHEMICAL CORROSION AND CAVITATION EROSION.

Jan. 5, 1971 SATQRU M|TQ ETAL 3,552,937

' SINTERED ALLOYS OF A CHROMIUM CARBIDE-TUNGSTEN CARBIDE-NICKEL- SYSTEMI I Fi1ed Feb-. 6 1969- 6 Sheets-Sheet 1 Cr3C2 so wc VICKERS HARDNESS(Hv) Jan. 5, 1971 SATORU MlTO ErAL 3,552,937

SINTERED ALLOYS OF A CHROMIUM CARBIDE-TUNGSTEN CARBIDE-NICKEL SYSTEM 6Sheets-Sheet 2 .Filed Feb. 6, 1969 FIG.2

FIG

Jan. 5, 1971- SATORU MITO ET AL 3,552,937

I v dlNl'EHEi) ALLOYS or A UHROMLUM CARUiDE-lUXUSl'lIN CARBIDE-NICKELSYSTEM Filed Feb. 6, 1969 6 Sheets-Sheet FIG; 4

INTENSIT IES DISTANCE ACC VOLTAGE i5KV CRYSTAL= LiF, LiF, KAP SAMPLECURR. OiZjJA DETECTOR Kr. ETA Kr. EXA, FPC

X-RAY=WLG CRKQ NiLCl CHART=20cm/min. SAMPLE 8,4.l/min.

| NTENSI TIES DiSTANCE ACC. VOLTAGE 15KV CRYSTAL= LiF, LiF, PbSC SAMPLECURR. 0.12pA DETECTOR= Kr.EXA, Kr.EXA, FPC X-RAY=WL(I CRKQ CKQ CHARTSPEED ZOmm/min.

, SAMPLE SPEED 8,u/min.

SATORU MITO ETAL SIXTEREU ALLOYS or A Jan. 5, 1971 CHROMIUMCARBIDE-TUXGSTEN (REL SYSTEM I 6 Sheets-Sheet 4 CARBIDE-N" Filed Feb. 6,1969 FIG. 7

EEBx

O w V. I5

ooo 0 v 6 4 2 60 TO Cr3C2 WC (WP/o) Fuss 0 20605 mo E395 TEST PERIOD(min) SATORU MITO ET AL SINTERED ALLOYS OF A CHROMIUM CARBID 3,552,937E-TUXGSTEN Jan, 5,1971

CARBIDE-NICKEL SYSTEM 6 Sheets-Sheet Q Filed Feb. 1969 US. Cl. 29182.7 1Claim ABSTRACT OF THE DISCLOSURE Sintered alloys mainly consisting of 25to 75 percent by weight of chromium carbide Cr C to 43 percent by weightof tungsten carbide WC and to 50 percent by weight of nickel Ni andcontaining one percent max. by Weight of impurities. These alloys arefully resistant to chemical corrosion and cavitation erosion.

The present invention relates to sintered alloys having prominentresistance to chemical corrosion and cavitation erosion.

Members exposed to a fast traveling fluid at elevated temperature aredamaged by chemical corrosion caused by oxidation as well as bycavitation erosion. Accordingly, such members are demanded to consist ofmaterials having great resistance to chemical corrosion and cavitationerosion. The best material now known is a cobalt-base alloy containingchromium, tungsten and nickel (commonly known as a Stellite alloy). TheStellite alloy had the drawback that since castings had to be machinedto obtain members of the desired shape and size, it was practicallyimpossible to prepare said alloy from a composition which would displaya hardness exceeding Generally, strength to cavitation erosion dependson the hardness of material, so that the Stellite alloy cannot be.deemed most suitable in this respect.

Other known materials having great hardness include sintered alloysconsisting of tungsten carbide and nickelchromium alloys.Metallographical1y, such alloy has a structure in which particles oftungsten carbide are dispersed in a matrix consisting of anickel-chromium alloy. This sintered alloy indeed has remarkablehardness and good mechanical strength due to the presence of dispersedparticles of tungsten carbide having great hardness. But the tungstencarbide is not fully resistant to chemical corrosion, so that thesintered alloy as a whole is not durable to damage resulting from othercauses than cavitation erosion.

The present invention provides sintered alloys mainly consisting of to75 percent by weight of chromium carbide, 10 to 43 percent by weight oftungsten carbide and 15 to percent by weight of nickel and containingone percent max. by weight of impurities. This sintered alloy has astructure in which particles of chromium carbide and those of tungstencarbide and bonded together by nickel and displays satisfactoryresistance to chemical corrosion and cavitation erosion at elevatedtemperatures. One of the other desirable properties of the sinteredalloy is that since it is mainly produced by powder metallurgicaltechnique, it is well adapted for machining. A material having suchsuitable properties is very valuable in application under severeconditions in which there often occurs cavitation erosion, for example,in the blades of a turbine for power generation or the nozzle of atemperature and pressure reducer.

United States Paten 3,552,937 Patented Jan. 5, 1971 The presentinvention can be more fully understood from the following detaileddescription when taken in connection with the accompanying drawings, inwhich:

FIG. 1 is the range of a composition of a alloy as specified by thepresent invention;

FIG. 2 is a 1200 fold magnified microscopic photograph of thecomposition of one alloy according to the invention;

FIG. 3 is a 1200 fold magnified microscopic photograph of thecomposition of another alloy according to the invention;

FIG. 4 is a chart derived from electron probe microanalysis, showing thedistribution of chromium, tungsten and nickel in the alloy of FIG. 3;

FIG. 5 is a similar chart to that of FIG. 4, showing the distribution ofchromium, tungsten and nickel in the alloy of FIG. 3;

FIG. 6 is a graph indicating the variation of hardness resulting fromdifierent contents of CI C +WC in a Cr C WCNi sintered alloy;

FIG. 7 is a graph showing the variation of transverse rupture strengthrelative to different contents of FIG. 8 is a graph denoting thevariation of erosion according to diiferent contents of Cr C +WC;

FIG. 9 is a graph comparing the erosion rate of the alloy of theinvention and that of several known alloys;

FIG. 10 is a graph showing the increased amount of oxidation due todifierent compositions of the alloy of the invention;

FIG. 11 is a longitudinal section of the nozzle of a temperature andpressure reducer fabricated according to an embodiment of the alloy ofthe invention;

FIG. 12 is a longitudial section of part of the adjusting valve for asteam turbine prepared from the alloy of the invention;

FIG. 13 is a perspective view of a lining of a turbine blade formed ofthe alloy of the invention;

FIG. 14 is a perspective view of a turbine blade to which is fitted thelining of FIG. 13; and

FIG. 15 is a longitudinal section of the reducer nozzle for supplyinggas turbine fuel prepared from the alloy of the invention.

The alloy of the present invention has a composition confined within therange defined by point A (40% Cr C 50% Ni and 10% WC) shown in thecomposition diagram of FIG. 1, point B Cr C 15% Ni and 10% WC), point C(42% Cr C 15% Ni and 43% WC) and point D (25% Cr C 50% Ni and 25% WC)and less than 1% of impurities based on the weight of the alloy.

The resistance to chemical corrosion of the alloy of the presentinvention substantially depends on the content of chromium carbide. Ifsaid content falls below 25 percent, the alloy will not displaysufiicient resistance to chemical corrosion. Conversely with a contentexceeding 75 percent, the alloy will be reduced in mechanical strengthdue to the resultant decrease in the nickel content.

Tungsten carbide itself helps the alloy to increase its hardness andpromotes the dispersion of fine particles of chromium carbide. Theminimum content of tungsten carbide to attain such effect is 10 percent.However, since tungsten carbide has lower resistance to chemicalcorrosion than chromium carbide, the maximum content of tungsten carbideallowable in an alloy required to have satisfactory resistance tochemical corrosion will be 43 percent.

Nickel plays the roll of a binder to bond together the particles ofchromium carbide and those of tungsten carbide. Small contents of nickellead to the increased hardness and decreased toughness of the alloy,While large amounts thereof bring about the reverse results. The optimumcontent of nickel ranges between and 50 percent.

The lower the purity of raw materials used, the lower will naturally bethe purity of the sintered alloy produced. Generally, chromium carbide,nickel and tungsten carbide contain iron, cobalt and molybdenumrespectively as impurities. Needless to say, the smaller their contents,the greater advantage will result. If the presence of these impuritiesonly accounts for 1 percent max. on the basis of the entire alloy, itseffect will be negligible.

The alloy of the present invention can be easily prepared by shaping andsintering a mixture of chromium carbide, tungsten carbide and nickeleach in powders using the ordinary techniques of powder metallurgy. Forinstance, the shaping of powdered raw materials may be made by pressmoulding, slip-casting, powder direct rolling or powder extrusion.Ashaped body of powdered raw materials having a prescribed shape andsize is preferably presintered under a non-oxidising atmosphere andfinally sintered at the liquid sintering temperatures of this alloy, andpreferably in a neutral or reducing atmosphere. These techniques ofpowder metallurgy are already known to those skilled in the art, and itwill be understood that the processes and conditions involved will notrestrict the present invention in any way.

The finally sintered product has a density equal to 98 readiness to besintered. Throughout the first and second sintering operations, theshaped body of powders displays a volumetric contraction of aboutpercent, so that as is known in this field, it is necessary to determinethe initial dimensions of the shaped body allowing for such shrinkage.

An example of the structure of a finally sintered alloy is illustratedby the microscopic photograph of FIG. 2. The alloy of FIG. 2 wascomposed of percent of chromium carbide, 30 percent of tungsten carbideand 20 percent of nickel and sintered 1 hour at a temperature of 1300 C.The surface was etched by an etching reagent prepared from the Murakamisolution. As clearly seen from FIG. 2, this sintered alloy has astructure consisting of particles of chromium carbide, smaller particlesof tungsten carbide distributed among the particles of the chromiumcarbide and a binder phase of nickel interposed between these particlesand bonding them together.

FIG. 3 shows the surface of a sintered alloy formed of percent ofchromium carbide, 30 percent of tungsten carbide and 15 percent ofnickel, which was etched in the same way as described above. As apparentfrom FIG. 3, this alloy also has substantially the same structure asthat of FIG. 2.

There was carried out electron probe microanalysis to determine thedistribution of the elements constituting this sintered alloy of thepresent invention which was formed of 55 percent of chromium carbide, 30percent of tungsten carbide and 15 percent nickel. FIG. 4 shows thestrength of the chromium contained in the chromium carbide, that of thetungsten contained in the tungsten carbide and that of the nickelforming a binding phase. The chart shows that in the region where theintensity of the chromium is reduced, the tungsten and nickel increasein intensity indicating that the sintered alloy consists of particles ofchromium carbide, a binder phase of nickel bonding together saidparticles and particles of tungsten carbide dispersed in said binderphase. FIG. 5 gives the strength of the chromium contained in thechromium carbide, that of the tungsten contained in the tungsten carbideand that of the carbon included in all the carbides. The chart showsthat there is present the tungsten carbide even in the region whichlacks the chromium carbide. Now referring to the strength of the carbon,the chromium carbide has a larger content of carbon than the tungstencarbide, so that the strength of the carbon varies according to thestrength of the chromium. This supports the fact that the chromium ispresent in the form of chromium carbide.

FIGS. 6 to 8 respectively show the Vickers hardness corresponding tochanges in the total content of chromium carbide and tungsten carbide ina CR C WCNi sintered alloy transverse rupture strength and extent oferosion. Throughout these figures, the zigzag lines 1 and 2 wereobtained by plotting the results of analysing several alloys having acomposition falling within the scope of the present inventioncorresponding to a line connecting points A and B and points on thedotted line a as shown in FIG. 1. The zigzag line 3 was plotted from theresults of analyzing several alloys having a composition outside of thescope of the present invention corresponding to points on the dottedline b of FIG. 1. The date of erosion is expressed in the reduced weightof alloy samples to be determined which were placed in boiling water andsubjected to vibrations having a frequency of 6100 c./s. and anamplitude of microns which were transferred through the water 150minutes by a cavitation tester using magnetostriction vibrations.

As seen from FIGS. 6 to 8, as the scope of the present invention isconcerned, increasing contents of carbides elevate the hardness of thealloy without widely varying the deflective strength thereof. Incontrast, alloys outside of the scope of the invention exhibit thegreater rate of erosion with increasing contents of carbides.

FIG. 9 gives the results of determining under the same conditions asdescribed above the rate of erosion displayed by the alloy of thepresent invention composed of 65 percent of chromium carbide, 20 percentof tungsten carbide and 15 percent of nickel, three known kinds ofStellite alloy and stainless steel. The figure clearly indicates thatthe alloy of the present invention has greater resistance to cavitationerosion than any of the known alloys.

FIG. 10 shows changes in the resistance to oxidation of the alloy of thepresent invention corresponding to the varied amounts of carbidecontained therein. Resistance to oxidation was determined by heating thesamples 5 hours at a certain temperature in the air and measuring anincreased weight over that present before said heating. The results showthat while the decrease in the content of chromium carbide, namely, therelative increase in the proportions of tungsten carbide and nickelindeed degraded the resistance to oxidation of the alloy as a whole,reduction in said resistance was practically negligible.

There will now be given some examples where there were tested samples ofthe alloy of the present invention under the conditions actuallyprevailing in manufacture as well as those in which the resultant alloywas subject to cavitation erosion.

EXAMPLE 1 There were mixed hours in a wet ball mill 55 parts by weightof powders of chromium carbide having an average particle size of about5 microns, 15 parts by weight of powders of nickel having a particlesize of 325 mesh max. and 30 parts by weight of powders of tungstencarbide having an average particles size of about 2 microns. To themixture were further added powders of paraflin. The mass was pressmoulded into a blind cylindrical body. The body was presintered 1 hourat a temperature of 600 C. in an atmosphere of dehydrated hydrogen 99.99percent pure (dew point 50 C.) and finally sintered 1 hour at atemperature of 1280 C. in the same atmosphere.

The finally sintered cylindrical body was machined into a nozzle 12, 75mm. in maximum diameter and mm. long shown in FIG. 11, which wasperforated with eight nozzle holes 8 mm. in diameter defining an angleof 45 degrees to the axial centre of the cylindrical body and arrangedat an equal space.

The nozzle 12 was fitted to a temperature and pressure reducer, andtested continuously for 50 days under the conditions where boiler steamat 500 C. and 200 atm., was reduced to 200 C. and 5 atm., respectivelyafter passing through said nozzle. The test confirmed that the nozzleshowed no unfavourable change, but was fully durable under the aforesaidconditions. By way of comparison, there was prepared from Stellite alloya nozzle having the same shape and size. This reference nozzle was putto test in the temperature and pressure reducer under the sameconditions as described above. A test continuously running for 30 dayscaused the end portion of said Stellite alloy nozzle to be deformed bywear, ceasing to display the initial performance.

EXAMPLE 2 There were mixed in a wet ball mill 50 parts by weight ofpowders of chromium carbide having an average particle size of about 5microns, 20 parts by weight of nickel having an average particle size of325 mesh max., 30 parts by weight of tungsten carbide having an averageparticle size of about 2 microns and a suitable amount of parafiinpowders. From the mass were prepared four rings having different sizesunder the same conditions as in Example 1. The rings were subjected topreand final sintering. The rings were fitted to the prescribed parts ofa steam adjusting valve for a steam turbine made of an alloy having acomposition of 1.25 percent of chromium, 1.0 percent of molybdenum, 0.2percent of vanadium, 0.19 percent of carbon and iron as the remainder.Referring to FIG. 12 illustrating the steam adjusting valve, numeral 21denotes a valve seat having a fluid passage 22, 23 an -O-ring made ofthe sintered alloy of the present invention fitted to the valve seat 21,24 a movable main valve rod for opening or closing the fluid passage 22,25 an O-riug mounted on the outer circumferential surface of the movablemain valve rod 24, 26 a movable auxiliary valve rod for opening orclosing a fluid passage 27 formed in the valve rod 24 and 28 and 29O-rings fitted to the parts of the main and auxiliary valve rods at'which they are brought into contact. These O-rings 23, 25, 28 and 29were silver brazed to the prescribed parts of the valve seat and rodapplying high frequency induction heating.

The steam adjusting valve fitted with said O-n'ngs was used continuouslyfor 500 hours in controlling the flow rate of steam at a temperature ofabout 500 C. running at the rate of 500 m./sec. The O-rings and brazedparts did not present any substantial damage. In contrast, steamadjusting valves of the same composition as previously mentioned buthaving the O-rings replaced by Stellite alloy D-2 welded to the parts towhich said O-rings were to be fitted, presented during a continuous testof about 120 r hours under the aforementioned conditions an increasedclearance between the valves due to the deformation and cavitationerosion of the welded parts..

EXAMPLE 3 There were mixed in a ball mill 130 parts by weight of powdersof chromium carbide having an average particle size of about microns, 40parts by weight of powders of nickel having a particle size of 270 meshmax. and 30 parts by weight of powders of tungsten carbide having anaverage particles size of about 1.5 microns. There were also added 1part by weight of ammonium alginate and small amounts of water. The masswas further adjusted in viscosity by addition of a 3 percent solution ofhydrochloric acid and 4 percent solution of caustic soda. The mass wasintroduced into a mould for fabricating a turbine blade lining. Afterbeing dried two days at normal temperatures, the moulded body wasfurther dried at a temperature of about 120 C., and then heated 1 hourat 1280 C. in a graphite boat packed with graphite powders in a hydrogenstream with the temperature progressively raised to said level at therate of 300 C. per hour. The sintered lining thus prepared had atheoretical density of 98.8 percent and and a hardness of Hv= 1350.

FIG. 13 presents the external appearance of said lining 31. As shown inFIG. 14, the lining was fitted to the ordinary turbine blade 32 andtested at a temperature of 118 to 120 C. and rotating velocity of 400m./sec. After the test was continued 500 hours, the lining made of thesintered alloy of the present invention did not present any undesirablechange. In contrast, a Stellite alloy lining displayed cavitationerosion in a test continued 68 hours under the same conditions, thesurface metal thereof losing its luster.

EXAMPLE 4 There were first mixed in a stainless steel pot 40 parts byweight of powders of chromium carbide having an average particle size of4.5 microns, parts by weight of powders of nickel having a particle sizeof 325 mesh max., and 30 parts by weight of powders of tungsten carbidehaving an average particle size of 1.7 microns. There was also added asuitable amount of acetone. After thorough mixing, the acetone wasremoved. The mass to which there were further added 2 parts by weight ofparaffin powders was pelletised into a form about microns thick using apelletiser. The pellet was rolled into a ribbon 0.63 mm. thick inaverage and 30 mm. wide using ahorizontal type roller. The ribbon waspresintered minutes at a temperature of 600 C. in an atmosphere ofhydrogen. The ribbon was cut into a turbine blade lining shaped as shownin FIG. 13. The lining was finally sintered 1 hour at a temperature of1270 C. The sintered alloy was contracted in size about 23 percent inthe rolling direction and had a theoretical density of 98.7 percent.

The lining was fitted to a turbine blade and tested at a temperature of120 C. and rotation velocity of 350 m./ see. as in Example 3. After thetest was continuously run 500 hours, the lining was proved to be freefrom any unfavourable damage.

EXAMPLE 5 There were mixed hours in a wet ball mill 50 parts by weightof powders of chromium carbide having an average particle size of 5microns, 25 parts by weight of powders of nickel having a particle sizeof 325 mesh max., and 25 parts by weight of powders of tungsten carbidehaving an average particle size of 1.7 microns. There were also addedabout 2 parts by weight of parafiin powders. The mass was press mouldedinto a plate 30 mm. wide, 300 mm. long and 10 mm. thick using a pressureof 1 to 1.5 ton/cm. The plate was presintered 1 hour at a temperature of600 C. in an atmosphere of dehydrated hydrogen 99.998 percent pure. Thispresintered material was cut into a turbine blade lining shaped .asshown in FIG. 13. The lining was finally sintered 1 hour at atemperature of 1280 to 1300 C. in the same atmosphere. The lining thusprepared was attached to a turbine blade, and tested continuously 500hours under the same conditions as in Example 4 and as a result wasproved to suffer substantially no damage.

EXAMPLE 6 There were fully mixed in a stainless steel pot 40 parts byweight of powders of chromium carbide having an average particle size of4.5 microns, 40 parts by weight of powders of nickel having a particlesize of 325 mesh max, 20 parts by weight of powders of tungsten carbidehaving an average particle size of 1.75 microns and a suitable amount ofacetone. After the acetone was removed, there was introduced a smallamount of a 5 percent aqueous solution of vinyl alcohol. The mass wasextruded into a plane plate 10 mm. wide and 3 mm. thick at the rate ofabout 1 mm./ sec. through a tool steel nozzle. The late was formed in awooden mould into a turbine blade lining as illustrated in FIG. 13.After being allowed to dry for two days, the lining was further dried 5hours at a temperature of C. Then the lining was finally sintered 45minutes at a temperature of 1280 C. in a graphite boat packed withgraphite powders. The sintered lining had a theoretical density of 99.0percent. After being fitted to a turbine blade, the lining was testedcontinuously 500 hours under the same conditions as in Example 4. Thesurface metal of the lining did not lose its luster, showing that thelining withstood the test.

EXAMPLE 7 There were mixed in a wet ball mill 45 parts by weight ofpowders of chromium carbide having an average particle size of microns,20 parts by weight of powders of nickel having a particle size of 270mesh max., and 35 parts by weight of powders of tungsten carbide havingan average particle size of 1.75 microns. The mixture was press mouldedinto a blind cylindrical body using a pressure of 1.5 ton/cm. and thenpresintered 1 hour at a temperature of 600 C. and a pressure of to 10torr units. After being cut into a reducer nozzle for supplying gasturbine fuel, the mass was finally sintered 1 hour at a temperature of1300 C. and a pressure of 10" to 10 torr units.

As shown in FIG. 15, the nozzle had a fluid passage 41 formed thereinand seventeen nozzle holes 42 extending from the fluid path 41 to theoutside in a direction intersecting the nozzle axis at right angles. Thesample nozzle was used continuously 5000 hours as a reducer nozzle forsupplying gas turbine fuel, but presented no cavitation erosion orchemical corrosion. Thus the nozzle displayed a useful life 10 times ormore longer than that prepared from the prior art nitride steel orStellite alloy.

What is claimed is:

1. A sintered alloy consisting essentially of 25 to 75 percent by weightchromium carbide, 15 to percent by weight nickel and 10 to 43 percent byweight tungsten carbide, and also containing impurities of less than 1percent based on the total weight of the alloy said chromium carbide andsaid tungsten carbide :being bonded together by said nickel.

Cemented Carbides, by Schwarzkopf V. Kietfer, 1960, pp. 197-198.

CARL D. QUARFORTH, Primary Examiner A. J. STEINER, Assistant ExaminerUS. Cl. X.R. 203

